The purpose of this study is to investigate in a quantitative and qualitative manner the nature of energy metabolism in the neonate. The investigation will be in two phases. Animal studies: A subhuman primate model (the neonatal and infant baboon) will be used for quantitative investigations of energy metabolism. Utilizing appropriately placed arterial and venous catheters for sampling of arteriovenous metabolite differences and the radioactive microsphere technique for measuring organ blood flow, cerebral, hepatic, renal, and muscle substrate uptake and production as well as oxygen consumption and carbon dioxde production will be evaluated. Clinical studies: Arterial and venous substrate levels in high risk infants and infants with hypoglycemia or hyperglycemia will be monitored prospectively as possible. Correlations with clinical condition will be made.